L12: Plain Carbon Steel

Question 1

What is an allotropic material?

Give an eg?

Answer 1

One with more than one crystal structure

Eg iron

Question 2

What affect do changes in crystal structure have on the metal?

Answer 2

Can affect the volume

Question 3

What are the diff phases in the iron- iron carbide phase diagram?

Answer 3

Alpha- ferrite- C in bcc Fe
Gamma- austerity- C in fcc Fe- 0.022wt%C

Sigma- sigma ferrite- C in bcc Fe- 2.14 wt%C

Fe3C - cementite- intermediate compound - specific composition- 0.09wt%C

Question 4

Why is there a difference in how much C can be encorparated in the fcc structure?

What does this lead to?

Answer 4

Size of interstitial sites in crystal structure.

Leads to vast range of properties obtained using simple heat treatments

Question 5

Define iron

Answer 5

Commercially pure
Less than 0.008 wt%C
Single phase 
V ductile
Slightly stronger than completely pure iron due to solid solution strengthening

Question 6

Define cast iron

Answer 6

Greater than 2.14wt%C

Question 7

Define steel

Answer 7

0.008wt% to 2.14wt%C

Usually less than 1%

Question 8

Define plain carbon steels

Answer 8

Only Fe and C

Trace quantities of P, Mn, Si added intentionally or accidentally in manufacture

Question 9

Define alloy steels

Answer 9

Fe and C with other elements added to obtain specific properties

Question 10

What is Pearlite and where is it formed?

Answer 10

Metal structure with grains composed of lamellae of alpha and Fe3C.
Approx 8:1 ratio of thickness

Pemex below eutectoid temp

Question 11

How does cooling rate affect the strength of pearlite?

Answer 11

Pearlite structure gives strength to alloy (dispersion hardening)

Slow cooling date gives more difFusion time- thicker lamellae

Faster cooling (not quenching) decreases layer thickness- more effective at preventing slip

Question 12

What is proeutectoid ferrite?

Answer 12

The alpha phase formed in the 2 phase region when the austenite starts to convert (tends to start at grain boundaries)

Question 13

What forms when 0.3wt% C is cooled?

Answer 13

Ferrite

Question 14

What forms when 1.5wt% C is cooled?

Answer 14

Cementite

Question 15

What happens to tensile strength with increased C%

Answer 15

Increases till 0.8wt% then decreases again

Question 16

What happens to hardness with increased wt%

Answer 16

Increases as a curve then begins to straighten out so gradient decreases with %

Question 17

What happens to elongation with increased C%

Answer 17

Decreases linearly then tends to zero at 0.9 ish

Question 18

What are the properties of hypoeutectoid alloys at room temp?

Answer 18

Pearlite islands surrounded by ferrite
Pearlite gives strength
Ferrite gives ductility
More C= more Pearlite= less strength and ductility

Question 19

What are the properties of hypereutectoid alloys at room temp?

Answer 19

Pearlite islands surrounded by cementite
Pearlite and cementite give strength
Cementite is brittle
Increased C= more cementite= more hardness but decrease in strength as more brittle